Spectral measurements play an important role in scientific research and technological applications for they afford information on the composition of matter under investigation and on the processes occurring therein on the molecular level. The wavelength region including the millimeter and far-infrared ranges is transitional between the radio-frequency and optical ranges and still remains most difficult for spectroscopic studies. This region has been handled by optical methods (diffraction and interference spectroscopy) as well as methods developed for the microwave range (mixing of the radiation of interest with that produced by a local oscillator based on a nonlinear element and measurement of the resulting signal at a lower intermediate frequency). Various spectral methods are comparable with respect to such basic parameters as the attainable spectral resolution, range of scanned frequencies, etc. The former two methods involve the use of grating and Fourier spectrometers which operate in the short-wave part of the range under investigation (at wave lengths shorter than 1 mm), permit continuous scanning of the spectrum (with interchanging of the optical components: gratings, beam splitters and others), provide for a resolution of 0.5 to 0.1 cm.sup.-1 (up to 0.05 cm.sup.-1 in the case of best laboratory models of Fourier spectrometers as opposed to commercially produced spectrometers), and are characterized by dimensions much greater than the wavelength of the radiation of interest (the higher the resolution, the greater the dimensions). Spectrometers using the principle of mixing the radiation of interest with that produced by a harmonic oscillator or a laser operate in the long-wave part of the range under investigation (longer than 1mm), provide for a resolution of up to 10.sup.-5 cm.sup.-1, but are not suitable for continuous scanning of spectra in a broad frequency range. Thus, the existing methods for spectral measurements in the millimeter and far-infrared ranges fail to provide means for continuous scanning of the spectrum in the entire region of interest with a sufficiently high resolution. Attempts to solve this problem using known methods invariably lead to serious complexity of the measuring procedure and instrumentation. The most advantageous method among the above methods of spectral studies in the millimeter and far-infrared ranges are offered by Fourier spectroscopy which has so far gained the widest acceptance.
Known in the art is a method for measuring the spectral distribution of electromagnetic radiation intensity (cf. "Spectroscopic Techniques for Far-Infrared, Submillimeter and Millimeter Waves", ed. D. H. Martin, North-Holland Publ. Co., 1967, Ch. 2. by converting the spectral distribution of electromagnetic radiation intensity to an electric signal using a direct integral transform, measuring the electric signal as a function of a direct integral transform parameter, and determining the desired spectral distribution of electromagnetic radiation intensity by applying an inverse integral transform to the measured function. In this method, the integral transform is a Fourier transform, the direct integral transform used in converting the spectral distribution of electromagnetic radiation intensity to an electric signal being based on the interference phenomenon with subsequent conversion of the power of the radiation under investigation to an electric signal, and the direct integral transform parameter is the optical path difference.
Due to the interference phenomenon the electric signal .DELTA.I depends on the path difference of rays .chi. and is correlated with the spectral density S(f) of the radiation being investigated by the integral Fourier transform according to the formula: ##EQU1## wherein .DELTA.I(o) is the signal at x=o, f is the radiation frequency, cm.sup.-1. In order to find a sought function S(f) to the measured dependence, use is made of the inverse Fourier transformation according to the formula: ##EQU2## (See, for instance, Spectroscopic Techniques, Ed. D. H. Martin, North Holland Publ. Co.-Amsterdam, 1967, Ch. 2, Sec. 2).
Also known is a spectrometer of millimeter and far-infrared ranges (cf. ibid.) for carrying out the above method, comprising the following components arranged in series along the electromagnetic radiation path: an electromagnetic radiation modulator and a unit for converting the spectral distribution of electromagnetic radiation intensity to an electric signal by means of a direct integral transform, a phase-sensitive detection unit whose one input is electrically associated with the output of the converting unit and whose other input is electrically associated with the modulator, a unit for applying an inverse integral transform to the electric signal measured as a function of a direct integral transform parameter, electrically associated with the phase-sensitive detection unit, and a control unit associated with the converting unit and electrically coupled to the input of the unit for applying an inverse integral transform to the measured function. In this spectrometer, the unit for converting the spectral distribution of electromagnetic radiation intensity to an electric signal by means of a direct integral transform comprises an interferometer optically associated with a broad-band radiation detector producing an electric signal varying directly with the power of the radiation passing through the interferometer, and the optical path difference is controlled in the interferometer by means of the control unit, by mechanical displacement of a movable mirror in the interferometer.
In the present spectrometer a unit for applying an inverse integral transform of the measured dependence of an electric signal on the direct integral transform parameter comprises a universal-type or a specialized computer for calculating the inverse Fourier transform according to Formula (2) as well as for collecting and processing data and displaying results (of R. J. Bell, Introductory Fourier Transform Spectroscopy, Acad. Press, No. 4, 1972, Ch. 18 (Introduction and Summary), Ch. 19 (Introduction and Summary), and publicity booklets of the companies manufacturing the Fourier spectrometers, for instance, Bruker-Physik AG.
However, the best resolution attainable by the above method and spectrometer is determined by the maximum path difference in the interferometer and a resolution of, for example, 0.001 cm.sup.-1 requires a path difference of 10 m, which is practically impossible to achieve because of the difficulties in designing the mechanical system for changing the path difference in the interferometer.
Besides, the prior art method and spectrometer necessitate, to obtain a maximum resolution, a precision path difference control system with an accuracy of better than 0.1 of the shortest wavelength in the spectrum under investigation, which complicates both the spectrometer and its operation and makes it difficult to attain high spectrum scanning speeds.
Moreover, the maximum resolution attainable by the prior art method and spectrometer is determined by the highest measured Fourier coefficient, which imposes more stringent requirements on the dynamic range of the broad-band radiation detector and synchronous detection unit because of the wide difference in the electric signal in different portions of the interference pattern.